Cystic fibrosis (CF) is a highly variable but inevitably fatal single gene disorder. Several lines of evidence suggest that genetic background contributes to the variability of CF phenotypes. We propose to develop CF as a model for the identification of modifier genes by capitalizing on the availability of a large motivated population of affected twins and siblings. The study has four aims: 1. To identify heritable CF phenotypes by twin study. Intrapair and interpair variance will be determined for selected CF phenotypes, and interclass correlations (MZ vs DZ) will be performed to identify CF phenotypes with a substantial heritable component. 2. To determine the contribution of genetic and other factors to the variability of CF phenotypes by analysis of affected sibs. Variance component methods will be used to evaluate the CF phenotypes that appear to be heritable based upon other studies or the results of aim 1. 3. To identify biologic phenotypes that correlate with heritable CF phenotypes by clinical study of twins and sibs. Multivariate analysis will be used to find biologic phenotypes associated with CF phenotypes. 4. To identify modifier genes and loci responsible for heritable CF phenotypes by linkage approaches. Identity by descent and transmission disequilibrium methods will be used to test linkage between candidate genes/loci and heritable CF phenotypes. To identify novel loci, genome-wide scans will be performed upon sib pairs selected for extreme concordance or discordance for heritable traits.